Plasmodium gametocytes circulate within red cells in the vertebtate blood stream in a state of developmental arrest. When infected blood is exposed to air the gametocytes are activated and form gametes within 10-15 min. Under these conditons activation is a bicarbonate-dependent event and requires a pH greater than 7.7. In the absence of bicarbonate, gametogenesis (exflagellation) can be induced by several pharmacological agents -phosphodiesterase inhibitors and A-23187, a divalent caction inophore. In addition, gametogenesis is initiated by a factor from the lumen of the mosquito midgut (mosquito exflagellation factor-MEF), the natural site of gametogenesis in Plasmodium. The study explores the intracellular changes involved in the activation of gametocytes of P. gallinaceum and the mechanisms whereby these changes occur. Experiments are designed to test a proposed model in which elevations in the intracellular pH, (cAMP), and (Ca ions) are normal events in activation. The study requires the refinement of a current technique to isolate mature gametocytes from othr parasite stages and from all host cell material in order to make direct measurements on cells. Such preparations will greatly facilitate ultrastructural studies of gametogenesis. Although gametocytes of many species of malaria parasites exflagellate spontaneously when exposed to air, it is unknown how many of these might require different stimuli when ingested by mosquitoes. The requirement for MEF for species other the P. gallinaceum will be determined and the range of mosquito species containing MEF will be exposed.